The overall enzymatic process generally adopted by manufacturers of high DX syrups from starch entails two-stages: liquefaction and saccharifications. The first step, the liquefaction, involves the hydrolysis of starch into a mixture of oligosaccharides, the so called maltodextrins. This process is catalyzed by alpha-amylases at a temperature of at least 75 C, preferably at about 90.degree. C. or by a jet-cooking process wherein the starch slurry is heated for at least several minutes to 105.degree.-110.degree. C., usually with a single dose of alpha-amylase, and then held at about 90.degree. C. for at least one hour.
A variety of microbial, particularly bacterial, alpha-amylases are commercially available for the liquefaction process, for example BAN.TM. (from Bacillus amyloliquefaciens and TERMAMYL.RTM. (from Bacillus icheniformis), both supplied by NOVO INDUSTRI A/S, Denmark, and THERMOLASE.TM. (from Bacillus stearothermophilus) available from Enzyme Development Corporation, N.Y., U.S.A. While BAN alpha-amylase is only stable up to about 85.degree. C. and hence barely suitable for the jet-cooking process, both the TERMAMYL and THERMOLASE enzymes are well adapted for this almost globally preferred mode of starch liquefaction because they are heat stable.
The saccharification step, in which the maltodextrins are converted into dextrose, is mostly catalyzed by a glucoamylase enzyme. Commercial glucoamylase preparations, usually derived from Aspergillus or Rhizopus species, are available from various manufacturers, e.g. as AMG.TM. 200L, a product obtained from Aspergillus niger and manufactured by NOVO INDUSTRI A/S, Denmark.
With a view to further increasing the dextrose yield from 30-40 percent by weight DS maltodextrin solutions it has become customary to conduct the saccharification process with glucoamylase in the presence of a debranching enzyme in order to facilitate the hydrolysis of branched oligosaccharides originating from the amylopectin portion of starch. One such debranching enzyme with maximum activity in the same pH and temperature ranges as glucoamylase is disclosed in European Patent Application No. 82302001.1 (Publication No. 0063909). The debranching enzyme is marketed by NOVO INDUSTRI A/S, Denmark, either as such under the proprietary name, PROMOZYME, or as a composition with suitable admixture of glucoamylase under the proprietary name DEXTROZYME.
Unfortunately, the otherwise very favorable combination of B. licheniformis alpha-amylase for liquefaction and glucoamylase-PROMOZYME for saccharification in the conversion of starch to high DX syrups entails an inconvenience. It has been observed that the presence of residual alpha-amylase activity from the liquefaction stage has a negative effect on the maximum DX obtainable by saccharification with glycoamylase-PROMOZYME. The problem is greatest with the thermostable B. licheniformis alpha-amylase which is still active at the preferred conditions for saccharification (of about pH 4.6 and temperature of about 60.degree. C., respectively). A remedy has been devised consisting of inactivation of the alpha-amylase prior to saccharification by acidification of the liquefied starch to a pH below 4.5 while maintaining a temperature of at least 90.degree. C. Following inactivation of the alpha-amylase, the temperature and pH are adjusted to saccharification conditions, meaning that the pH has to be brought up to about 4.5. This additional pH adjustment inevitably increases the salt content of the syrup and hence the expenses connected with de-salting the final syrup.
The object of the present invention is to overcome the above-mentioned inconveniences still associated with the use of B. licheniformis alpha-amylase for the conversion of starch into a high DX syrup. This and other objects which will be dealt with subsequently in this specification are attained by conducting the liquefaction process with a novel type of alpha-amylase.